1. Field of the Invention
The present invention relates to a rate adaptive pacemaker of the type having an arrangement for determining the demand of the patient's organism, a pacing rate control for controlling the pacing rate in response to the patient's demand, and a pacing rate limiter for preventing the pacing rate from becoming too low.
2. Description of the Prior Art
The pacing rate of a rate adaptive pacemaker may become too low due to the physical demand of the patient's organism and heart. This may result in lack of oxygen supply to the myocardium. Under certain conditions the heart may not be able to fulfil the physiological needs of the patient's organism and heart if the pacing rate is not limited.
It is known to set a lower limit for the pacing rate. This limit value is normally determined from the patient's diagnosis and a constant or externally programmable limit can be set. Thus U.S. Pat. No. 4,535,774 describes a stroke volume controlled pacemaker, in which the heart rate is permitted to range between prescribed minimum and maximum heart rate values. Further, in U.S. Pat. No. 5,861,011 a pacemaker is disclosed having a system for determining the circadian rhythm by examining variations in the QT interval and adjusting the pacemaker night time setting of a lower rate limit to the lower value than the pacemaker daytime setting of the lower rate limit. In U.S. Pat. No. 5,183,040, an antitachycardia pacer is disclosed which analyzes cardiac output information for assessing hemodynamic status and determining adequate blood supply.
Thus, a pacing rate that is too low may cause an influx of blood enriched with oxygen that also is too low. A prescribed suitable lower pacing rate limit avoids the slow influx of the fresh blood. At the same time this lower limit value should be low enough so as to not to disturb a peaceful sleep. In that case the patient can feel more healthy in various everyday life conditions including peaceful sleeping.